1. Field of the Invention
The present invention generally relates to a display device, and particularly to a display device employing an electrophoretic liquid material referred to as electrophoretic ink which utilizes the electrophoretic phenomenon. This display device is also a storage medium known as electronic paper or an electronic sheet.
2. Description of the Related Art
The phenomenon of charged particles dispersed in a liquid electrophoresing by applying an electric field; in other words, electrophoretic phenomenon, is well known from conventionally. As an application of this phenomenon, known is the fact that by dispersing charged pigment particulates in a dispersed liquid colored with dye, sandwiching these between a pair of electrodes, and applying an electric field thereto, the charged particles are drawn to one of the electrodes, and there have been attempts at applying this to displays. Here, the result of dispersing charged particles in a dispersed liquid colored with dye is referred to as “electrophoretic ink” or “electrophoretic liquid material” and a display employing the above is referred to as an “electrophoretic display (EPD)”.
TiO2 (rutile structure), for example, is used as the core of the charged particles, and polyethylene, for example, is used as the cover layer for covering this core. As the solvent, for instance, used is a solvent in which anthraquinone dye is dissolved in ethylene tetrachloride and isoparaffin. The charged particles and solvent have mutually differing colors, and, for example, the charged particles are white, and the solvent is blue, red, green or black. At least one of the electrodes is a transparent electrode.
When an external electric field is applied to the electrophoretic ink (hereinafter simply referred to as “electronic ink”) in the electrophoretic display, the charged particles move in the direction opposite to the direction of the electric field when the charged particles are negatively charged. Thereby, the one face for viewing the electronic ink; that is, the surface of the display, appears to be colored in either the color of the solvent or the color of the charged particles. Therefore, by controlling per pixel the movement of the charged particles of the electronic ink positioned in the respective pixel areas, it becomes possible to express display information such as characters, symbols and images on the display surface.
As the relative density of the solvent and charged particles is set to be approximately the same, the charged particles are able to retain the position at the time of electric field application even after the disappearance of the electric field for a relatively long period of time (e.g., from several minutes to about twenty minutes). Thus, low electric consumption can be expected upon being applied to a display.
Moreover, the aforementioned EPD has an extremely wide viewing angle at ±90 degrees, and the contrast is also high. In addition, with a typical EPD, the observers will as a result be viewing the color of the pigment or dye, and, for example, this is different than viewing the light of a fluorescent tube which is a backlight in a transparent liquid crystal display, and realizes a coloration gentle to the eyes. Cost reduction is further possible.
In reality, however, reliability could not be secured due to the agglomeration of the aforementioned pigment particulates, and could not be put into practical use for a long time. Nevertheless, as it became apparent in recent years that reliability could be improve by filling electronic ink in microcapsules, EPDs were catapulted into the spotlight.
As a specific display utilizing this electronic ink, known are the theses “44.3L: A Printed and Rollable Bistable Electronic Display—P. Drzaic et al., SED 98 DIGEST 1131” and “53.3; Microencapsulated Electrophoretic Rewritable Sheet”—H. Kawai et al., SID 99 DIGEST 1102”.
The former proposes a flexible display wherein a transparent conductor plate, capsuled electronic ink, patterned silver or graphite conductor layer, and insulation film layer are successively printed in this order on polyester film, and, in order to designate an address of the patterned conduction layer, a hole is provided in the insulation film layer and a lead line is provided therein. The later proposes a rewritable sheet pursuant to electrophoresis employing microcapsuled electronic ink, and a method for writing on this sheet.
Nonetheless, the aforementioned EPDs still have the following unresolved problems. In other words, with electronic ink as described above, the relative density of the solvent and charged particles is set to be approximately the same, and the charged particles are thereby able to retain the position at the time of electric field application even after the disappearance of the electric field for a relatively long period of time. That is, although it possesses a retention property capable of functioning as a memory for retaining data, the time thereof is merely several minutes to several ten minutes. Thereafter, the position of the charged particles moves in the solvent, and the picture quality of the display pattern gradually deteriorates.
Moreover, for instance, with the likes of a liquid crystal display, regardless of the screen changing or not, rewriting is conducted at a speed of sixty times per second. Upon employing this driving method, however, it is not possible to make the best use of the retention property in the aforementioned electronic ink, and low energy consumption cannot be realized thereby.
Further, information represented by the electronic ink is basically two values (two values pursuant to whether the color appearing on the display surface by the charged particles moving is of the charged particles or the solvent), and a fine gradation cannot yet be expressed in a sufficient manner.
Accordingly, an object of the present invention is to retain the function of maintaining information displayed by the electronic ink and preventing the deterioration of the picture quality of the display information in a display device using the electrophoretic phenomenon of a liquid material.
Another object of the present invention is to enable the rewriting of display contents of pixels to be of a required minimum upon renewing the display contents and realizing low energy consumption in this type of display device.
A further object of the present invention is to enable the expression of fine gradation in this type of display device.